Null Exciton-Coupled Chromophoric Dimer Exhibits Symmetry-Breaking Charge Separation

Abstract

A comprehensive understanding of the structure− property relationships in multichromophoric architectures has pushed the limits for developing robust photosynthetic mimics and molecular photovoltaics. The elusive phenomenon of null exciton splitting has gathered immense attention in recent years owing to the occurrence in unique chromophoric architectures and consequent emergent properties. Herein, we unveil the hitherto unobserved null exciton coupling assisted highly efficient photoinduced symmetry-breaking charge separation (SB-CS) in a Greek cross (+)-oriented spiro-conjugated perylenediimide dimer (Sp-PDI 2 ). Quantum chemical calculations have rationalized the infrequent manifestation of null exciton coupling behavior in Sp-PDI 2 . Negligible contribution of long-range Coulombic and shortrange charge-transfer mediated coupling renders a monomer-like spectroscopic signature for Sp-PDI 2 in toluene. The Greek cross (+)-arranged Sp-PDI 2 possesses a selective hole-transfer coupling, facilitating the ultrafast dissociation of null excitons and evolution of the charge-separated state in polar solvents. Radical cationic and anionic spectroscopic signatures were characterized by employing femtosecond transient absorption spectroscopy. The substantial hole transfer electronic coupling and lower activation energy barrier of Sp-PDI 2 accelerated the charge separation rate. The rate of charge recombination (CR) markedly decelerated due to falling into the inverted region of the Marcus parabola, where the driving force of CR is larger than the total reorganization energy for CR. Hence, the ratio of the rates for SB-CS over CR of Sp-PDI 2 exhibited an unprecedently high value of 2647 in acetonitrile. The current study provides impeccable evidence for the role of selective charge filtering in governing efficient SB-CS and thereby novel insights towards the design of biomimics and advanced functional materials.

Linker dependent symmetry breaking charge separation in 9,10-bis(phenylethynyl)anthracene dimers

“…It has been found that the SB-CS rate and the recombination rate is strongly sensitive to the electronic coupling between the adjacent molecular units that is determined by the relative inter-/intra-molecular distance and orientation. 13,18–21 For example, Wasielewski group investigated the SB-CS process in cofacial and linear perylene diimide (PDI) dimers and found that ultrafast SB-CS (1/0.52 ps −1 ) could be conducted in cofacial PDI dimer even in weakly polar toluene but with a fast recombination rate (1/222 ps −1 ). 22 Instead, SB-CS cannot proceed in linear PDI dimer in toluene but only in polar solvents.…”

Synthesis and Ultrafast Broadband Optical Limiting Properties of a Two-Branched Twistacene

“…The negative signal centered at 458 nm is ascribed to the superposition of stimulated emission signal with ESA ( Figure 2 ). During the entire delay time, the curve shape of ESA remains the same with a time depend decay of OD, indicating that the ESA originates from an excited singlet state and there is no additional transition process In addition, the decay of extracted dynamics traces at 458 nm is similar to that at 662 nm ( Figure S1 ), suggesting that the ESA is derived from the fluorescent states and there is no additional transition process occurring [ 33 , 34 , 35 ]. This ESA behavior is appreciated in ultrafast OL.…”

Vibronic Resonance Along Effective Modes Mediates Selective Energy Transfer in Excitonically Coupled Aggregates

“…Interestingly, the interference between vibronic couplings described here is not phase dependent and always leads to larger overall couplings. This is distinctly different, for example, from the case of null excitons 44 where charge-transfer and Coulomb couplings can interfere destructively. The phase-independent interference between vibronic couplings manifests in more general contexts.…”